The invention is directed to a process for the preparation of an aqueous colloidal precious metal suspension, as well as to the preparation of a supported precious metal catalyst, using the said suspension.
Metal colloids, more in particular precious metal colloids, are used as starting materials for the preparation of supported (precious) metal catalysts. Precious metal colloids are usually prepared by reducing a precious metal ion in an organic solvent, mostly at elevated temperature.
The most often used method is reduction using an alcohol. This can either be a low boiling alcohol, such as the C1 to C4 alcohols, more in particular methanol, or high boiling solvents containing a hydroxyl group, such as ethylene glycol, or diethylene glycol mono-n-butyl ether. When low boiling alcohols are used, a separate stabilizer is added. The metal salt (ion) is mixed with the stabilizer in an alcoholic solution and refluxed for several hours. The high boiling materials require very high temperatures and often a protective atmosphere (nitrogen) and/or a high pH.
There are some methods which do not require the use of organic solvents or additives, such as the use of citrate as reductant and stabilizer. The lower limit of the particle size is, however, about 5 nm. It is difficult, or even impossible to produce nanoparticles of lower particle size.
The use of borate as reducing agent has the disadvantage of high pH and a very cumbersome feed method, if one needs a narrow particle size distribution. This method is already difficult at laboratory scale, but impossible to operate at industrial scale. The same applies to the use of 3-acetic acid thiophene as reducing and stabilizing agent.